In recent years, liquid crystal substances have been applied to display media utilizing the reversible motion of liquid crystal molecules such as display elements represented by TN (twisted nematic) type and STN (supertwisted nematic) type liquid crystal display elements. Besides these applications, these liquid crystal substances have been studied for application to optically anisotropic films such as optical phase retardation plate, polarizing plate, light polarizing prism and light filter taking advantage of its anisotropy in physical properties such as olientability, refractive index, dielectric constant and magnetizability.
In such an optically anisotropic film comprising a liquid crystal substance as a constituent component, it is essential that the uniform alignment of the liquid crystal be semipermanently fixed to obtain stable and uniform optical properties.
As a method for semipermanently fixing the uniform alignment of the liquid crystal, there has already been known a method which comprises aligning a liquid crystalline compound containing a polymerizable functional group or a polymerizable liquid crystal composition containing such a compound in liquid crystalline state, and then irradiating the material with energy rays such as ultraviolet ray while maintaining in the liquid crystalline state.
As such a technique there may be exemplified one employing the following liquid crystalline compound.
For example, JP-A-58-102205 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses as a liquid crystal compound containing a polymerizable functional group a compound represented by the following formula: ##STR1##
However, the foregoing compound is disadvantageous in that it exhibits a nematic phase at a temperature as very high as between 108.degree. and 211.degree. C. and an optically anisotropic film (color polarizing plate) prepared by the photopolymerization of a polymerizable composition containing such a compound in liquid crystal state actually shows a nonuniform external appearance, causing unevenness.
JP-A-4-140921 discloses an optically anisotropic film prepared by a process which comprises the polymerization of a bifunctional acrylate compound represented by the following general formula (R-2): ##STR2## wherein R.sup.5 represents a hydrogen atom or a methyl group as a liquid crystal compound having a polymerizable functional group to form a polymer network. The foregoing patent also discloses a technique for obtaining an optical compensation plate (phase retardation plate) suitable for STN liquid crystal display by adding a chiral compound to the polymerizable composition of the general formula (R-2) to introduce a helical structure of the mesogenic core of the liquid crystal into the optically anisotropic film.
The use of the compound of the general formula (R-2) has advantages of mechanical strength and heat resistance. However, as exemplified in the foregoing patent, a liquid crystal composition made of 80 parts by weight of a compound of the general formula (R-2) wherein R.sup.5 is a methyl group and 20 parts by weight of a compound of the general formula (R-2) wherein R.sup.5 is a hydrogen atom exhibits a nematic phase at a temperature as relatively higher than room temperature as between 80.degree. and 121.degree. C., and an optically anisotropic film prepared from such a polymerizable liquid crystal composition disadvantageously exhibits nonuniformity of the orientation of mesogenic core of the liquid crystal, also due to inducing the undesirable heat polymerization.
The foregoing method for fixing the alignment of liquid crystal is disadvantageous in that the liquid crystalline phase of the polymerizable liquid crystal compound or polymerizable liquid crystal composition is in a relatively high temperature range, inducing photopolymerization by energy ray as well as undesirable heat polymerization that causes the loss of uniform orientation of the liquid crystal molecules. Thus, nonuniform orientation of the resultant polymer different from the desired orientation is obtained.
In this respect, it is necessary that the polymerizable liquid crystal or polymerizable liquid crystal composition has liquid crystalline temperature range in the vicinity of room temperature and that photopolymerization of the polymerizable liquid crystal or polymerizable liquid crystal composition is carried out in the vicinity of room temperature to avoid the progress of undesirable heat polymerization.
In order to solve the foregoing problem, JP-A-62-70406 discloses the use of a compound represented by the following general formula (R-1): ##STR3## wherein f represents an integer 2, 5 or 6; and R.sup.4 represents a hydrogen atom or a methyl group as a liquid crystal compound containing a polymerizable functional group.
The foregoing patent has no definite reference to the phase transition temperature of the liquid crystal compound used. But, the foregoing patent discloses that photopolymerization at 50.degree. C. can provide a uniform optically anisotropic film.
However, the optically anisotropic film obtained by the photopolymerization of the compound of the general formula (R-1) is disadvantageous in that it has a low mechanical strength. The optically anisotropic film thus obtained is also disadvantageous in that when heated to around 100.degree. C., it loses a fixed uniform orientation, restricting its use as an optically anisotropic film.
The approach disclosed in the foregoing patent is further disadvantageous in that all the liquid crystalline acrylates exhibit a monotropically liquid crystalline phase and thus tend to undergo crystallization during photopolymerization, making it difficult to obtain an optically anisotropic film in which a uniform orientation of the mesogenic core of liquid crystal, which is fixed by photopolymerization.
Moreover, JP-A-4-227611 discloses liquid crystalline acrylate compositions containing a compound represented by the following general formula (R-3): ##STR4## wherein R.sup.6 represents a hydrogen atom or a methyl group; and g represents an integer 5, 6, 8, 9, 10, 11 or 12. All these compositions only exhibit a monotropically nematic phase as mentioned above. Most of these compositions immediately undergo crystallization at room temperature. These compositions exhibited a stable nematic phase, if any, for only one day at longest.
As another method for semipermanently fixing the uniform orientation of the mesogenic core of liquid crystal, there has already been known a method which comprises the use of a liquid crystalline polymer compound.
In some detail, the foregoing method comprises applying a solution of liquid crystalline polymer which exhibits a thermotropic liquid crystallinity to a substrate which is treated to align the liquid crystalline polymer, and then subjecting the material to heat at a temperature at which the liquid crystalline polymer compound exhibits a liquid crystalline phase to obtain a desired orientation of mesogenic core of liquid crystalline polymer. The polymer compound thus oriented is kept in glass state so that the desired orientation is fixed therein. JP-A-4-3022 discloses that a compensation plate having a homogeneous orientation structure can be used as a color compensation plate for STN liquid crystal display. JP-A-5-27235 discloses that a compensation plate having a homeotropic orientation structure can be used as a compensation plate for viewing angle dependence of TN and STN liquid crystal display. JP-A-5-61039 discloses that a compensation plate having a cholesteric orientation structure can be used as a compensation plate for the viewing angle dependence of TN liquid crystal display.
Further, an approach is known which comprises the use of a film having a thickwise refractive index greater than inplane refractive index (optically anisotropic film) as a compensation plate to reduce the.viewing angle dependence of liquid crystal display element (M. Akatsuka et al., "Japan Display", 1989, page 363). As such an optically anisotropic film there is disclosed one comprising a liquid crystalline polymer compound in JP-A-5-27235 and JP-A-5-34678.
However, these compensation plates are disadvantageous in that the orientation structure in the liquid crystalline polymer compound is fixed in glass state. Thus, the orientation structure is destroyed at temperatures higher than the glass transition temperature of the liquid crystalline polymer compound. Accordingly, the working temperature is restricted to not more than the glass transition temperature of the liquid crystalline polymer compound. Further, since the viscosity of liquid crystalline polymer compounds is higher than that of low molecular liquid crystalline compounds, it takes much time to obtain a desired uniform alignment, reducing the productivity. The greater the area of the desired compensation plate is, and the higher the glass transition point of the desired liquid crystalline polymer compound is, the more remarkable is this disadvantage.
The liquid crystalline polymer compound, if used, is applied to the substrate which is treated to align liquid crystal in the form of solution in a solvent. Thus, this approach cannot be applied to substrates having a poor solvent resistance such as some plastics. Accordingly, this approach is disadvantageous in that the substrates employable are restricted by the solvent used.
Since the materials of the optically anisotropic film which has heretofore been used are limited to those described above, there arises a problem in the method for the preparation of an optically anisotropic film from these materials.
As mentioned above, neither optically anisotropic films having an excellent mechanical strength and heat resistance and good uniformity and practical value which can be obtained with a good producibility nor polymerizable liquid crystal compositions which exhibit a liquid crystalline phase in the vicinity of room temperature and can be used to obtain such optically anisotropic films have been known.
In the field of optically anisotropic film for liquid crystal display, it has been keenly desired to develop an optical compensation plate having a chiral compound-based helical structure introduced thereinto to enhance the display quality of liquid crystal display. In the field of optoelectronics such as optical logic device, it has been keenly desired to develop an optical element comprising a polymer film with a controlled orientation structure which exhibits excellent uniformity and heat resistance to facilitate the reduction of the weight of the apparatus and the rise in the area of the apparatus.